Are They Worthwhile?
Wind turbines installed on rooftops have the potential to provide renewable energy at a reasonable cost. Do the sums add up to the same total?
There is a private high school located not too far from where I work that has several little wind turbines on the top of the building. A neighbor of mine has installed a little wind turbine on the roof of his garage. My neighborhood is quite windy. In addition, each morning on my walk to work, I pass a farmhouse with a little wind turbine installed on a tower thirty feet tall. How much power can be produced by wind turbines mounted on rooftops? Let’s figure it out together.
Power Rating
Wind turbines often have a rated power that is listed beside them. Small turbines, such as the kind you can see on a roof, typically have ratings ranging from 400W to 1kW. Therefore, you might do a fast calculation in your head and speculate that the 1 kW turbine would produce 24 kWh of energy each day (1kW x 24 hours.) If the wind were to blow continuously at the speed specified in the rating, then your statement would be correct. However, the reality is that none of those circumstances will likely take place on a rooftop. To begin, the rated power of a turbine represents the greatest possible outcome. It is a measurement of how much power the turbine will produce at the wind speed that is considered the absolute maximum that the turbine can withstand. Examining the turbine’s power curve is one way to get a more precise estimate. The following is an example of a power curve for a turbine with a capacity of 1 kW:
The graph demonstrates that the cut-in speed for the turbine is around 3 meters per second (6.7 miles per hour), which is when it begins producing electricity. Less strong winds cannot provide enough power to turn the rotor. There is a quick rise in power as wind speed increases, but the power output does not reach the specified output of 1 kW until the wind speed is around 11 meters per second (nearly 25 mph). To put it into perspective, if the typical wind speed over your property was 25 miles per hour, every single one of your trees would be permanently bowed. It is more probable that you will experience winds in the range of 3 to 5 m/s, implying that a 1 kW turbine typically generates less than one-tenth of its rated value. This is because the wind speed ranges from 3 to 5 meters per second. The speed at which the turbine will engage a braking mechanism to protect itself from being damaged is referred to as the shut-down speed. Because a typical shut-down speed is just a few milliseconds per second faster than the rated speed, the “sweet spot,” or the range of speeds in which the turbine generates its rated power, is a very small window.
Sustained Winds Vs the Turbulent Winds
In order for wind turbines to provide cost-effective electricity, there must be access to strong and consistent winds. A residential rooftop provides none of these amenities. The top of a home is quite near to ground level, which is problematic since wind speed rises with increasing height. Even more problematic, all of the obstructions, including trees, other buildings, and even the home itself, cause the wind to become turbulent. You don’t receive a rapid steady breeze flowing in a largely consistent direction as you would expect; rather, you get brief choppy gusts of wind from various directions. Not only does turbulence reduce the turbine’s output, but it also generates mechanical stress, which shortens the turbine’s lifespan. The general rule of thumb is that the wind turbine need to be at least 9 meters (30 feet) taller than any impediment that is within 150 meters (500 feet):
I just came across a research that suggested rooftop turbines should be positioned closer to the center of the roof rather than around the roof’s perimeter. The reasoning behind this recommendation was that turbulence is stronger around the roof’s edge than in the center. In all fairness, though, that research focused on turbulence, not the entire output. If you follow the manufacturer’s instructions, your turbine will have a longer lifespan. This is a fortunate development since it is possible that it will be a very long time before the wind turbine generates enough power to cover the turbine’s cost. A more sound recommendation would be to forego the installation of wind turbines on the roof.
Payback Period
We undertake a calculation exercise to determine the payback time of many different types of tiny wind turbines in the introductory class that I teach on sustainable energy. We went with a horizontal axis wind turbine (HAWT) rated at 400 watts and a 1 kW HAWT, although I won’t name any specific manufacturers here. And because higher elevations are where you get the sustained winds, we opted for a 10 m (33 ft) tower rather than installing it on the rooftop since it would have been a waste of time. In the winter, the average wind speed at a height of 10 meters in northern Illinois is 3.6 meters per second, which is equivalent to around 8 miles per hour.
400 Watt HAWT
The 400W wind turbine only produces 50W when the wind speed is 3.6 meters per second. Assuming that it operates nonstop for the whole year, the turbine will produce 438 kilowatt hours of electricity. The cost of one kilowatt-hour of power in the United States is now $0.12, meaning that the turbine owner will save $52 on their electric bill. A standard wind turbine with 400 watts of power will cost you approximately $400, and that’s just for the turbine itself, not including the tower. The tower kit that I could locate for the lowest price retails for little under $400; however, this price does not include the concrete foundation. Therefore, the base price for one of these turbines and its tower is somewhere in the neighborhood of 800 dollars. That is a payback time of more than 15 years, which wouldn’t be horrible for a long-term investment if the turbine didn’t come with a one-year guarantee included in the product’s purchase price.
1 kW HAWT
At a wind speed of 3.6 meters per second, a well-known turbine with a rating of one kilowatt will produce 77 watts of electricity. This results in an annual savings of 675 kWh for the consumer, which equates to an annual savings of $81 on their power bill. The tower is not included in the price of the turbine, which is $4400, and since the turbine is more heavier, it needs a more robust tower. In most cases, the cost of a tower is about equivalent to the cost of the turbine, which results in an investment that is very near to $8800. If Bilbo Baggins’ parents had purchased one of these on the day he was born, it would have paid for itself by the time he was eleventy-one years old. Oh, I didn’t even begin to touch on the expense of having a professional install it, which could be upwards of $2,000 in certain cases.
What Are Your Alternatives If You Can’t Use the Grid?
When compared to electricity from the grid, it’s simple to conclude that these turbines aren’t worth the money, but what about distant areas that don’t have access to grid power? Let’s have a look at the turbine and tower that are 1 kW and cost $8800. According to our calculations, it generates 675 kWh annually. Is there a sustainable source of energy that is superior than this one? Solar? The solar resource at the same site receives an average of 4.5 peak sun hours (PSH) each day, with the worst case scenario receiving 2.6 PSH during the winter months. For a photovoltaic (PV) array to create 675 kWh each year, it would need to generate 1.85 kWh daily. Even if the total system were only 75% efficient, a 1 kW solar array would be sufficient to cover that during the winter months. (A more accurate estimate would be 85 percent.) Because the cost of installing a modest PV system is around $4 per watt, the entire expenditure, including installation, is approximately $4000. Even better, the photovoltaic system does not have any moving components, so it does not need yearly maintenance. When I do the statistics, I simply can’t fathom why a homeowner would choose a modest wind turbine when solar photovoltaics (PV) is the superior alternative. I just don’t get it. Wait… I guess I can come up with one: There is a sister-in-law of mine that resides in the great state of Alaska!
Wind: Be Powerful or Don’t Bother
Wind energy is an excellent renewable energy source at the scale of utility production. Large turbines are more effective than their smaller counterparts, and taller towers are better at capturing the high-speed winds that contain the most energy. If you are thinking of purchasing a tiny turbine, you should reconsider. If you live somewhere other than in the Arctic, you would be better off investing the money on solar panels.